tui: add startup auto-submit diagnostic logging

Headed swarm spawns hand off the initial prompt via a client-input file
that the resumed TUI auto-submits once remote history loads. When that
submit was deferred (history not loaded, queued dispatch pending, still
processing) or skipped (empty input), there was no trace, making
'prompt seen but never sent' cases hard to debug.

Add logs at three points:
- server persist_headed_startup_message: confirms handoff file written
- apply_restored_reload_input: confirms restore + submit flag decision
- process_remote_followups: logs each distinct deferral reason once (via
  startup_submit_deferred_reason guard) and logs when submit fires/fails

No behavior change; logging only.